Ascertainment of the Effect of Compressive Loading on the Fatigue Lifetime of Graphite Epoxy Laminates for Structural Applications.

Abstract
The main objective of this research investigation was the experimental determination of the effect of compressive loading on the fatigue response of graphite/epoxy lamiantes. The primary emphasis of the program was on the accumulation of a statistically significant data base. The data base was to be used for developing an analytical model with predictive capability to account for the effect of compressive loading. The test program included only unnotched coupons in a room temperature, laboratory air environment. The test program consisted of static tensile and compressive tests; tension-tension and tension-compression fatigue tests; and tensile and compressive residual strength tests of coupons tested either under tension-tension or tension-compression fatigue loading. The development of a large, statistically meaningful data base was accomplished. The general effect of compression on the fatigue response of a graphite/epoxy composite was determined. However, a general model with capability to account for compressive loading was not attempted. As an alternate, the concept of a fatigue threshold was postulated along with consideration of some substantiating data. Analysis of the data showed that due to the extent of data scatter, a model of the 'wear-out' or similar type would predict the need for unacceptably low stress levels to achieve the high probabilities of survival required for aircraft structures.

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